What is the main idea of this paragraph?
The Missile Gap was in essence a growing perception in the West, especially in the U.S.A., that the Soviet Union was quickly developing an intercontinental range ballistic missile (ICBM) capability earlier, in greater numbers, and with far more capability than that of the United States. Even as that perception was disproved, it became evident that the Soviets were placing their major effort toward developing strategic missiles against which, once launched, there was no defense. The perceived missile gap that ensued was based on a comparison between U.S. ICBM strength as then programmed, and reasonable, although erroneous estimates of prospective Soviet ICBM strength that were generally accepted.